Fuel cell system with a combined fuel evaporation and cathode gas heater unit and its method of operation

ABSTRACT

Fuel cell system with a combined fuel evaporation and cathode gas heater unit, and its method of operation A fuel cell system, in which the cathode gas heater and the evaporator are combined in a single compact first heat exchange unit which includes a first housing inside which thermal energy is transferred from the first coolant to both the cathode gas and the fuel.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the U.S. national phase of PCT Application No.PCT/DK2020/050275 filed on Oct. 8, 2020, which claims priority to DKPatent Application No. PA 2019 70650 filed on Oct. 17, 2019, thedisclosures of which are incorporated in their entirety by referenceherein.

FIELD OF THE INVENTION

The present invention relates to fuel cell systems, for example HT-PEMfuel cells, with a burner and a reformer and a heat exchange system andits use for a vehicle as well as methods of operating such fuel cellsystem.

BACKGROUND OF THE INVENTION

When generating electricity with fuel cell systems, also heat isgenerated as a by-product, which is removed by cooling-liquid that iscirculating through channels in the fuel cell. The temperature isadjusted by flow of cooling-liquid, for example based on glycol, throughheat exchangers and radiators for optimized function of the fuel cell.On the other hand, the coolant can be used for heating the fuel cellsduring startup conditions.

Three way heat exchangers are known in general for coolant circuits. Anexample for three cooling circuits through a single three way heatexchanger is disclosed in Korean patent application KR2017-0087077.However, three way heat exchangers have not yet been proposed for heatexchange between liquid and gases for providing compact solutions forfuel cell systems, especially in automobiles. Instead, two-way crossflow heat exchangers are typically used for fuel cell systems as heatexchangers for air or as condensers.

WO 2019/158173 A1 describes a fuel cell system comprising a stack offuel cells, each including a cathode, an anode and a polymerelectrolyte, and a cooling fluid circuit. The fuel cell system furthercomprises a cathode gas supply feeding cathode gas to the cathodes, anda reformer receiving evaporated fuel and converting it to syngas whichis fed to the anode side of the fuel cells. The fuel cell system alsocomprises a reformer burner heating the reformer, an evaporatorreceiving liquid fuel and evaporating it to form evaporated fuel whichis fed to the reformer. The evaporator is heated by the cooling liquidof the cooling circuit.

WO2017/148487 discloses a fuel cell system with a cooling circuit inwhich the coolant is split into a major portion that passes through aheat exchanger for evaporating fuel and a minor portion that passesthrough a heat exchanger for cooling of the coolant. US2015/0340715discloses a fuel cell system in which the coolant is used for heatingand humidifying air and hydrogen before the air and hydrogen enter thefuel cell module. US2001/0019788 discloses a cooling circuit wherecoolant is used for evaporating fuel.

For fuel cell systems, especially in the automobile industry, there is asteady demand for optimization. In particular, there is a demand forminimization of space and weight. This above-mentioned prior art doesnot teach that the cathode gas heating system and the evaporator arecombined in a single compact first heat exchange unit which comprises ahousing and which is configured for transfer of thermal energy from thecoolant inside the housing to both the cathode gas and the fuel.

DESCRIPTION/SUMMARY OF THE INVENTION

It is an objective of the invention to provide an improvement in theart. In particular, it is an objective to provide a fuel cell systemthat is compact and useful for being provided in little space, forexample in automobiles. This objective and further objectives areachieved with a fuel cell system and method as described in thefollowing and in the claims.

As set forth in the following, different principles are presented forachieving compactness, while optimizing the thermal efficiency.

The fuel cell system comprises a fuel cell, typically a fuel cell stack.Herein, the term fuel cell is used for a single fuel cell as well as formultiple fuel cells, for example a fuel cell stack.

The fuel cell comprises an anode side and a cathode side and a protonexchange membrane therein between for transport of hydrogen ions fromthe anode side to the cathode side through the membrane duringoperation.

For example, the fuel cell is of the type that operates at a hightemperature. The term “high temperature” is a commonly used andunderstood term in the technical field of fuel cells and refers tooperation temperatures above 120° C. in contrast to low temperature fuelcells operating at lower temperatures, for example at 70° C. Forexample, the fuel cell operates in the temperature range of 120-200° C.

For example, the fuel cell in the fuel cell system is a high temperaturepolymer electrolyte membrane fuel cell, (HT-PEM), which operates above120 degrees centigrade, differentiating HT-PEM fuel cell from lowtemperature PEM fuel cells, the latter operating at temperatures below100 degrees, for example at 70 degrees. The normal operating temperatureof HT-PEM fuel cells is the range of 120 to 200 degrees centigrade, forexample in the range of 160 to 170 degrees centigrade. The polymerelectrolyte membrane PEM in the HT-PEM fuel cell is mineral acid based,typically a polymer film, for example polybenzimidazole doped withphosphoric acid. HT-PEM fuel cells are advantageous in being tolerant torelatively high CO concentration and are therefore not requiring PrOxreactors between the reformer and the fuel cell stack, why simple,lightweight and inexpensive reformers can be used, which minimizes theoverall size and weight of the system in line with the purpose ofproviding compact fuel cell systems, for example for automobileindustry.

The fuel cell is used to create electricity, for example for driving avehicle, such as an automobile. In order to provide a buffer for theproduced electricity, typically a battery system is provided inelectrical connection with the fuel cell.

For example, air is used to provide oxygen to the fuel cell. In thiscase, an air supply is provided for supplying air to the cathode side.Prior to entering the fuel cell, the air is heated by an air heatingsystem for increasing the temperature of the air. The air provides theoxygen for the fuel cell. Other gases of the air merely flow through thesystem and are discarded again.

As an alternative to air, in principle, another gas that contains oxygengas or even pure oxygen gas could be used. For simplicity, the gas thatcontains oxygen is herein called cathode gas, be it pure oxygen gas,air, or other gas blends that contain oxygen, which is provided to thecathode side of the fuel cell. Correspondingly, the gas heating systemis generally called a cathode gas heating system.

A first cooling circuit is provided for recirculating first coolantthrough the fuel cell for adjusting the temperature of the fuel cellwith the first coolant. During normal operation, the first coolingcircuit is taking up heat from the fuel cell in order to keep thetemperature stable and in an optimized range. For example, thetemperature of the fuel cell is 170 degrees, and the first coolant has atemperature of 160 degrees at the entrance of the fuel cell.

For example the first coolant is glycol based. However, in case that thefuel cell system is used for automobiles in cold areas, glycol is notoptimum for the start-up, why other liquids are preferred. Examples ofsuch other liquids include synthetic oils.

A reformer with a catalyzer is used for catalytic conversion of fuelinto syngas, which contains the necessary hydrogen gas for the anodeside of the fuel cell, which is used in the fuel cell for production ofelectricity. Accordingly, the reformer is conduit-connected to the anodeside of the fuel cell. The reformer comprises a catalyzer inside areformer housing, which has reformer walls.

For the catalytic reaction in the reformer, the provided liquid fuel isevaporated in an evaporator that is conduit-connected on its downstreamside by a fuel vapor conduit to the reformer. The upstream side of theevaporator is conduit-connected to a liquid fuel supply for receivingfuel, for example a mix of liquid methanol and water.

For heating the reformer to the proper catalytic conversion temperature,for example in the range of 250-300 degrees, a reformer burner isprovided and in thermal contact with the reformer for transfer of heatto the catalyzer inside the reformer. The reformer burner comprises aburner-chamber providing flue gas by burning anode waste gas or fuel orboth. For example, the reformer burner provides flue gas at atemperature in the range of 350-400 degrees, is provided from a reformerburner.

For example, in normal operation, the flue gas from the reformer burneris passing along the reformer walls and heats them. In such embodiment,the burner-chamber is in fluid-flow communication with the reformerwalls for flow of the flue gas from the burner-chamber to and along thereformer walls for transfer of heat from the flue gas to the reformerwalls.

After the transfer of the thermal energy from the flue gas to thereformer walls, remaining thermal energy can be used for heating othercomponents, for example batteries that are used to store the electricalenergy of the fuel cell, or for heating a vehicle cabin. The reformerburner is configured for providing flue gas by burning anode waste gasor fuel or both.

For example, the reformer and reformer burner are provided as a compactunit. Optionally, in order to provide one way of a compactburner/reformer unit, the reformer walls are tubular and surround theburner walls. However, this is not strictly necessary, and a serialconfiguration, or a side-by-side configuration of the burner/reformer ora configuration of a burner sandwiched between two sections of thereformer is also possible.

For compactness, advantageously, the cathode gas heating system and theevaporator are combined in a single compact first heat exchange unitcomprising a first housing and which is configured for transfer ofthermal energy from the first coolant inside the first housing to boththe cathode gas and the fuel, so that both the cathode gas and the fuelreceive thermal energy from the first coolant.

In this case, a first stream exchanges heat with to other streams, forexample counter-flowing streams. More than three streams are availablefor the first and further heat exchange units.

For example, the first heat exchange unit comprises two heat exchangemodules which receive the first coolant serially or in parallel. Thelatter principle yields a higher efficiency, allowing a more compacttechnical solution.

Different principles are possible for the multi-stream heat exchangeunit. For example, the coolant is diverted into two or more heatexchange areas or heat exchange modules. This implies that a first partof the coolant is flowing into one of the heat exchange areas or modulesand only, or at least predominantly, exchanging energy with fluid inthat heat exchange area or module, while a second part of the coolant isflowing into another of the heat exchange areas or modules and only, orat least predominantly, exchanging energy with fluid in that other heatexchange area or module. Alternatively, the multi-stream heat exchangeunit is of the type in which the coolant flows in a space that issandwiched between multiple heat exchange modules. The heat is in thiscase transferred from the first coolant in between the modules to thefluids in the respective module. For example, the multi-stream heatexchange unit has a three-layer configuration.

In some practical embodiments of the first heat exchange unit, thecathode gas heating system is provided as a first heat exchange module,and the evaporator is provided as a second heat exchange module, andthere is provided a space inside the housing, the space being sandwichedbetween the first and the second heat exchange module.

Optionally, it is configured for flow of the first coolant in the spacefor transferring heat from the first coolant to both the first and thesecond heat exchange module simultaneously on either side of the space.

As options, bent configurations, for example cylindrical configurations,are possible, but also unbent configurations. Optionally the first andthe second heat exchange module are provided in parallel with respect toeach other with the space for the flow of the coolant in between.

In other practical embodiments, the first heat exchange unit isconfigured for diverting at least a part of the first coolant that flowsinto the first housing into a first portion of the first coolant foronly or predominantly exchanging energy with the cathode gas and asecond portion of the first coolant for only or predominantly exchangingenergy with the fuel.

Optionally, the first heat exchange unit comprises a first and a secondheat exchange module, wherein the first heat exchange module isconfigured for receiving only the first portion of the first coolant,and the second heat exchange module is configured for receiving only thesecond portion of the first coolant.

In some practical embodiments, the first and the second heat exchangemodule are provided with a distance in between for minimizing thermalconduction from the first to the second heat exchange module.

In practical embodiments, there are provided at least three flow pathsin the first heat exchange unit, the three flow paths comprising a firstflow path, a second and a third flow path through the first heatexchange unit. The first flow path is for the first coolant and is partof the first cooling circuit. The second flow path is for the cathodegas and constitutes the cathode gas heating system. The third flow pathis for flow of the fuel and constitutes the evaporator for the fuel.

However, this results in a complex heat exchange principle, where theheat exchange with the cathode gas influences the heat exchange with thefuel. Therefore, as an alternative for a configuration that is simplerto control, the first heat exchange unit comprises two heat exchangemodules with the second and third flow path, respectively, each of whichis connected to the first flow path for thermal exchange to the firstcoolant. However, also in this case, all heat exchange modules for theheat exchange between the first coolant and the cathode gas as well asthe fuel are provided in the same first heat exchange unit and includeexchange heat with the first coolant inside the first housing.

In a practical embodiment, the first heat exchange unit has a firstinlet for inflow of the first coolant into the first housing and a firstoutlet for outflow of the first coolant from the first housing. Betweenthe first inlet end the first outlet, there is provided the first flowpath inside the first housing for the first coolant, the first flow pathconnecting the first inlet with the first outlet.

Additionally, the first heat exchange unit and has a second inlet andsecond outlet for the second flow path, which is for the cathode gas,and the second flow path constitutes the cathode gas heating system. Thefirst heat exchange unit has a third inlet and a third outlet for athird flow path, which is for flow of the fuel. The third flow pathconstitutes the evaporator for the fuel. Optionally, further flow pathsare provided.

In particular embodiments, the first inlet is conduit-connected to thecoolant downstream side of the fuel cell for receiving coolant from thefuel cell after the coolant has traversed the fuel cell. The secondinlet is conduit-connected to the cathode gas supply, for example airsupply, for receiving oxygen gas for the cathode side of the fuel cell.The second outlet is pipe connected to the cathode side of the fuel cellfor providing heated oxygen gas to the fuel cell. The third inlet isconduit-connected to the fuel supply for receiving fuel, for example amethanol/water mix. The third outlet is connected to the anode side ofthe fuel cell for supplying evaporated fuel to the anode side of thefuel cell.

For example, the first flow path is in thermal connection with thesecond and the third flow path for simultaneous transfer of thermalenergy inside the first heat exchange unit from the first coolant to thecathode gas and from the first coolant to the fuel for heating the fueland cathode gas during their flow through the first heat exchange unitwhen the fuel cell system is in operation.

The flow paths are separated from each other by thermally conductingwalls, typically metal walls.

When the fuel cell system is in power producing operation, a flow of thefirst coolant is provided through the first flow path, a flow of thecathode gas through the second flow path, and a flow of the fuel throughthe third flow path. Simultaneous transfer of thermal energy is achievedfrom the first coolant to the cathode gas for heating the cathode gasand from the first coolant to the fuel for evaporating the fuel.

In particular it is emphasized that the transfer of heat to the secondand third flow path is advantageously not provided in two seriallyconnected heat exchange modules because it is a disadvantageefficacy-wise in that the coolant would change temperature when flowingthrough a first heat exchange unit before reaching the second, whichreduces efficacy. For higher efficacy, the principle is rather aparallel principle of simultaneous heat exchange from one medium to twoothers, although the flow itself need not be in parallel motion.

In some practical embodiments, a least a part of the inflow of firstcoolant through the first inlet into the first flow path is divertedinto a first and a second portion, where the first portion of the firstcoolant is only or predominantly exchanging energy with the cathode gasin the second flow path, and the second portion of the first coolant isonly or predominantly exchanging energy with the fuel in the third flowpath. Optionally, the first and second portion of the first coolant arecombined again prior to flowing out of the first outlet of the firstheat exchange unit.

The principle of a multiple stream heat exchanger, for example three wayheat exchanger, is also applicable for a different function as explainedin the following. This principles can be used alternatively to the firstheat exchanger or additionally.

Optionally, a second multi-stream heat exchange unit is provided as asecond single unit with a second housing and comprising at least threeflow paths, the three flow paths comprising a fourth, a fifth and asixth flow path through the second heat exchange unit. The fourth flowpath is for a second coolant different from the first coolant and partof a second cooling circuit, which is different and flow-separated fromthe first cooling circuit. The fifth flow path is for the first coolantand part of the first cooling circuit. The sixth flow path is for atleast one of:

a) the exhaust gas from the cathode side and

b) the flue gas from the reformer burner.

Typically, the sixth flow path is used for both a and b, and thecorresponding gases are combined upstream of the sixth flow path. Thefourth flow path is in thermal connection with the fifth and the sixthflow path for simultaneous transfer of thermal energy from the firstcoolant to the second coolant for reducing the temperature of the firstcoolant, and from the exhaust gas to the second coolant for condensingthe water of the exhaust gas in the sixth flow path prior to leaving thefuel cell system through an exhaust.

In a practical embodiment, the second heat exchange unit has a fourthinlet for inflow of the second coolant and a fourth outlet for outflowof the second coolant. Between the fourth inlet end the fourth outlet,there is provided the fourth flow path for the second coolant, thefourth flow path connecting the fourth inlet with the fourth outlet.

Similarly, the second heat exchange unit comprise a fifth inlet andfifth outlet for the fifth flow path, which is for the first coolant.The second heat exchange unit additionally has a sixth inlet and a sixthoutlet for a sixth flow path, which is for flow of the exhaust gas.Optionally, further flow paths are provided.

In some practical embodiments, a least a part of the inflow of thesecond coolant through the fourth inlet and into the fourth flow path isdiverted into a first and second portion of the second coolant, wherethe first portion of the second coolant is only or predominantlyexchanging energy with the first coolant in the fifth flow path, and thesecond portion of the second coolant is only or predominantly exchangingenergy with the exhaust gas in the sixth flow path. Optionally, thefirst and second portion of the second coolant are combined again priorto flowing out of the second heat exchange unit.

In operation, when the fuel cell system is in power producing operation,flow of the second coolant is provided through the fourth flow path andflow of the first coolant through the fifth flow path. Through the sixthflow path, there is provided flow for at least one of the exhaust gasfrom the cathode side and the flue gas from the reformer burner.Simultaneous transfer of thermal energy is achieved from the firstcoolant to the second coolant for reducing the temperature of the firstcoolant, and from the exhaust gas to the second coolant for condensingthe water of the exhaust gas.

The fact that the heat exchange units work in a multi-stream flowprinciple makes adjustment of the correct temperatures more complex ascompared to using two different heat exchangers. However, by usingtemperature gauges and flow meters controlled by a correspondinglyprogrammed electronic controller, the flow can be readily adjusted andprecisely controlled by a correspondingly programmed logical feedbackcontrol system during operation. The advantage of the multi-stream heatexchange unit is a far more complex technical solution, requiring verylittle space in the fuel cell system of a vehicle, such as automobile.

Optionally, the three paths in the first heat exchange unit areseparated by thermal conducting metal walls, and optionally the firstflow path is sandwiched between the second and the third flow pathwithout thermal conduction from the second to the third flow path.Similarly, as an option, the three paths in the second heat exchangeunit are separated by thermal conducting metal walls, and optionally thefourth flow path is sandwiched between the fifth and the sixth flow pathwithout thermal conduction from the fifth to the sixth flow path.

For example, the first multi-stream heat exchange unit is a counter-flowheat exchanger. Similarly, as an option, the second multi-stream heatexchange unit is a counter-flow heat exchanger.

In some embodiments, the system comprises the first as well as thesecond heat exchange unit.

Optionally, the system further comprises a water separator that isfluid-flow connected to the sixth flow path between the secondmulti-stream heat exchange unit and the exhaust, and the water separatoris configured for separating at least part of the condensed water fromthe gas flowing from the sixth flow path towards the exhaust.

In some practical embodiments, the liquid fuel supply comprises analcohol, optionally methanol, reservoir for supplying the alcohol.Additionally, a water supply is provided for supplying water and formixing the water with the alcohol at a mixing point upstream of theevaporator.

For example, the water supply is configured for supply of water that isrecycled from the flue gas of the burner. Accordingly, as a furtheroption, the water separator is part of a water recycling system forproviding water to alcohol prior to evaporation of the mix of alcoholand water as fuel.

In some useful embodiments, the system comprises a startup heater forheating the fuel cell system during startup conditions prior to normalpower producing fuel cell operation. During startup of the fuel cellsystem, the fuel cell has to be heated up for reaching a steady stateelectricity-producing state. Especially for use in vehicles, thestart-up procedure should be fast. Typically, this is done in practiceby transferring the heat from the flue gas to the first coolant in thefirst cooling cycle which during start-up is used as heating fluid,instead, in order to heat up the fuel cell to a temperature suitable fornormal power producing operation.

Optionally, the first cooling circuit comprises a dedicated branch forflow of the first coolant through the startup heater in startupconditions, wherein the dedicated branch is connectable in startupconditions for flow of the first coolant from the startup heater throughthe fuel cell and through the first multi-stream heat exchange unit forheating the fuel cell, the cathode gas and the fuel. Optionally, thededicated branch comprises flow of the first coolant only in startupconditions but not during steady state power producing operation of thefuel cell.

SHORT DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to thedrawing, where

FIG. 1 illustrates a flow diagram of an example of a fuel cell systemwith a triple flow heat exchange unit;

FIG. 2 illustrates the coolant flow in start-up situation;

FIG. 3 illustrates the coolant flow in normal operation during powerproduction;

FIG. 4 is an illustration of an example of a multi-stream heat exchangeunit, wherein

FIG. 4 a shows two different perspective views, FIG. 4 b illustrates asemi-transparent, partially cut-away view, and FIG. 4 c a side viewinside the unit;

FIG. 5 is an illustration of an alternative example of a multi-streamheat exchange unit, wherein FIG. 5 a is a perspective view, FIG. 5 b isa semi-transparent view, FIG. 5 c a partial cut-away view into the unit,FIG. 5 d illustrates a top with two sectional views.

DETAILED DESCRIPTION/PREFERRED EMBODIMENT

FIG. 1 is a flow diagram of a fuel cell system 1 in which a first heatexchange unit 21 and a second heat exchange unit 22 are used forremoving heat from the fuel cell. The two heat exchange units 21, 22 areexemplified as multi-stream heat exchange units 21, 22, for examplethree-way heat exchange units. However, it is also possible that onlyone of them is employed. A multi-stream heat exchange unit is a compactsolution which is highly advantageous in automobiles, where the space isscarce.

In the left of FIG. 1 , a methanol supply tank 2 is shown, whichdelivers methanol 3 through corresponding tubes 4, including a branchthrough tube 4 a for supply of methanol to a startup heater 5 includinga valve 6 for the start-up heater 5, branches through tubes 4 b and 4 cfor supply of methanol for reformer burners 7, as well as a branchthrough tube 4 d for supply of methanol as part of the fuel 20 to thereformers 8 after having been mixed with water 9 at a mixing point 10.

In the reformer 8, the mix of methanol CH₃OH and water H₂O iscatalytically converted into hydrogen gas H₂ and CO₂. Simplified, themethanol CH₃OH is converted into 2H₂ and CO, and the water moleculesplits into H₂ and CO₂, where the oxygen is captures by the CO toproduce CO₂. The mix of H₂ and CO₂ is then supplied as so-called syngas11 to the anode side 12 of the fuel cell 15, typically fuel cell stack.Air 16 from the environment 17 is supplied to the cathode side 14 of thefuel cell 15 in order to provide the necessary oxygen for the reactionwith the hydrogen to produce water, after hydrogen ions H+ have passedthe membrane from the anode side 12 to the cathode side 14.

Advantageously, the fuel cell 15 is a high temperature polymerelectrolyte membrane (HT-PEM) fuel cell. Typically, high temperaturefuel cells operate in the temperature range of 120-200° C. For example,the fuel cell 15 operates at a temperature of 175° C.

This operation temperature is held constant by a correspondinglyadjusted flow of first coolant in a first cooling circuit 18 through thefuel cell 15. For example the temperature of the first coolant at thecoolant inlet 19 of the fuel cell 15 is in the range of 160° C. to 170°C.

During the flow cycle of the first coolant in the first cooling circuit18, the temperature of the first coolant has to be reduced to thetemperature desired at the coolant inlet 19 of the fuel cell 15. Forachieving this temperature reduction, the first coolant flows through afirst heat exchange unit 21.

The first heat exchange unit 21 is exemplified as a multi-stream heatexchange unit, in particular a three way heat exchange unit, whichsimultaneously transfers heat between different fluids that flow throughthe first heat exchange unit 21. In FIG. 1 , also the second heatexchange unit 22 is exemplified as a multi-stream heat exchange unit, inparticular with at least three streams, which transfers heatsimultaneously between different fluids that flows through the secondheat exchange unit 22.

In contrast to two serially connected heat exchangers where coolant isfirst exchanging thermal energy with one fluid and then, downstreamthereof, exchanging thermal energy to a second fluid, the multi-streamheat exchange unit provides a simultaneous exchange of thermal energybetween the fluids. Although, the flow direction of the fluids insidethe heat exchange unit can be counter-crossing and not in parallel, theprinciple itself for the heat exchange is according to a parallelprinciple, for example a side-by-side flow, in contrast to a serialprinciple. In a serial principle, one heat exchanger is placed after theother, so that the coolant first flows through a first heat exchanger,and then the same coolant flows through a second heat exchanger.

In the first heat exchange unit 21, the direction of the first coolantis opposite to the flow of the fuel and the air, which are colder thanthe first coolant. In the second heat exchange unit 22, the direction ofthe second coolant is opposite to the flow of the first coolant and theexhaust gas, which are warmer than the second coolant during normaloperation.

Notice that the exemplified temperature of 175° C. of the first coolantat the coolant outlet of the fuel cell 15 is not much different from thetemperature of 160° C. to 170° C. of the first coolant at the inlet tothe fuel cell 15. This implies that only a relatively small amount ofthermal energy has to be removed from the first coolant cycle 18. Theconsequence thereof is that the heat exchange units 21, 22 can beconstructed relatively small. This is one reason why high temperaturefuel cells are useful for compact systems, especially when used inautomobiles.

In the current configuration, the first coolant in the first heatexchange unit 21 transfers heat to the methanol/water mix as fuel 20 foruse in the reformers 8 in order to cause it to evaporate into a fuel gasso that the methanol/water fuel 20 can be supplied to the reformers 8 asa gas mix. The first coolant also transfers heat to the air 16 that issupplied into the cathode side 14. The heat transfer from the firstcoolant to the methanol/water fuel 20 and the air 16 ensures that thegases have a temperature high enough for efficient reaction.

The fact that the first heat exchange unit 15 works in a multi-streamflow principle makes adjustment of the correct temperatures for the fuel20 and the air 16 more complex as compared to using two different andseparate heat exchangers. However, by using temperature gauges, valves35, 36, and flow meters controlled by a correspondingly programmedelectronic controller, the flow can be readily adjusted and preciselycontrolled by a correspondingly programmed logical feedback controlsystem during operation. However, the advantage of the first heatexchange unit 21 is a far more compact technical solution, requiringvery little space in the fuel cell system of a vehicle.

As explained above, some of the heat from the fuel cell is removed inthe first heat exchange unit 21. In order to remove heat in general fromthe system, a second heat exchange unit 22 is employed. This second heatexchange unit 22 transfers heat to a second cooling circuit 25. Thesecond coolant circuit 25 removes thermal energy from the fuel cell 15and is cooled in a cooler with a radiator 26, for example as it isnormally known for vehicles in which an engine has to be cooled.

FIG. 1 , and as emphasized in FIG. 3 , illustrates the flow in the firstcoolant circuit 18 during normal operation. The coolant flows from thefuel cell 15 through the first heat exchange unit 21 along coolantcircuit branch 18A, passing branch point 39, then to the pump 38, andfrom there into the fuel cell 15 again. At branch point 40, a portion ofthe first coolant, for example not more than 50%, is diverted into asecond branch 18B through valve 36 and into the second heat exchangeunit 22 in order to transfer heat to the second cooling circuit 25.

As already discussed above for the exemplified temperatures for hightemperature fuel cells, there is not much different from the inlettemperature of 160° C. to 170° C. and the outlet temperature of 175° C.This implies for the second branch 18B that only a minor portion of thefirst coolant, for example in the range of 5-50% or 5-30%, has to bediverted at branch point 40 and flow through the second branch 18B. Dueto the fact that only a minor portion of the first coolant flow throughbranch 18B between branch points 40 and 39, as compared to the amount inthe branch 18A, not only the second heat exchange unit 22 can be ofrelatively small size, but also the valve 36 and the tubing of branch18B can be made small and compact. This is a great advantage for systemsthat require compact and light-weight solutions such as automobiles.This is one reason why high temperature fuel cells are useful forcompact systems, especially when used in automobiles.

Potentially, the second heat exchange unit 22 is also a multi-streamheat exchange unit. One of the multiple streams through the secondmulti-stream heat exchange unit 22 is for the first coolant that alsohas one way through the first multi-stream heat exchange unit 21. In thesecond multi-stream heat exchange unit 22, the first coolant is reducedin temperature by heat exchange with a second cooling circuit 25. Thesecond coolant cycle 25 is also taking up thermal energy from theexhaust gas 27 from the reformer burner 7 and from the fuel cell cathodeside 14.

The exhaust gas contains water vapor, nitrogen gas, carbon dioxide, andoxygen gas. Due to the cooling in the second heat exchange unit 22, thewater can be condensed in a liquid/gas separator 28. From the liquid gasseparator 28, an amount of water is condensed in a water storage tank 29for recycling for mixing with methanol 3 at the mixing point 10, while aremaining part of the steam and condensed water is discarded through theexhaust 30 together with the other gases.

The cooling of the water vapor is advantageous in that the water vapouris not leaving the exhaust 30 as hot steam at a temperature that couldcause injury for people near the exhaust 30 pipe but rather leaves thesystem primarily as condensed water, drip-ping out of the exhaust pipe.This is a pronounced advantageous side effect of the system, in additionto its compactness.

For the air supply 16 to the cathode side 14 of the fuel cell 15, acompressor 31 is used, which is also cooled by the second coolantcircuit 25. This second cooling circuit 25 is optionally also used forcooling further equipment, such as current converters 32 to ramp downthe voltage from the fuel cell 15 stack and converters 33 that changedirect current (DC) voltage to alternating current (AC) voltage forpower supply to the compressor 31.

In order to ensure clean methanol and water supply, correspondingfilters 34 a, 34 b are, respectively, applied in the methanol supplytank 3 and in the recycle connection for the water between the waterseparator 28 and the mixing point 10.

The fuel cell system 1 is optionally provided with only one of the twomulti-stream heat exchange units 21, 22. Alternatively, furthermulti-stream heat exchange units are employed. The two shownmulti-stream heat exchange units 21, 22 are examples of how these can beemployed in different ways.

In start-up situations, which is shown in FIG. 1 and emphasized in anexample in FIG. 2 , methanol and air are burned in start-up heater 5,which heats up the first coolant in a third branch 18C of the firstcooling circuit 18. From this third branch 18C, the first coolant isthen flowing from the startup heater 5 through the fuel cell 15 in orderto heat the fuel cell up to operation temperature.

Downstream of the fuel cell 15, the first coolant, which in start-upsituations is a heating fluid, flows also through the first multi-streamheat exchange unit 21 in order to heat the incoming air 16 and causeevaporation of the methanol/water mix fuel 20. Once, operationtemperature of the fuel cell 15 has been reached, the coolant flow isadjusted to the normal operation in the first cooling circuit 18, fortemperature control of the fuel cell, as illustrated in FIG. 3 .

The amount of first coolant through the startup heater 5 is adjusted tothe amount needed to achieve a sufficiently high temperature. This isadjusted with an adjustment valve 35. The transition between start-upflow for the coolant and the flow for normal operation is done byinterplay between the adjustment valve 35 in the start-up branch 18C forthe coolant and the adjustment valve 36 for the coolant in the normaloperation branch 18B when in normal operation conditions. The normaloperation branch 18B is emphasized in FIG. 3 .

Typically, during normal operation, the startup heater 5 is not working,and the flow in the third branch 18C closed by closure of valve 35. Inturn, valve 36 is open during normal operation but closed during startupor at least in the initial phase of the startup. During the start-upheating, the resulting gas 37 from the startup heater 5, which includeswater, carbon dioxide and nitrogen gas, is also fed through the secondheat exchange unit 22 in order to reduce the temperature before beingled out of the exhaust 30 pipe without the risk for injury of persons.Possible water separation by the water separator 28 already in thisstartup stage ensures that there is a water supply for the methanolwater mix as fuel 20 for reformer 8 and the fuel cell 15.

The fact that water is recycled and also produced during start-up addsto the compactness of the system in that no large water storage tank 29is needed. It is also important to notice that reduced or possibly evenavoided storage of water is an advantage in that problems with freezingwater in the system at very low environmental temperatures are avoided.

In FIG. 1 , a number of other components are illustrated, such astemperature gauges, pressure gauges and additional valves, which howeverare not explained in detail herein. These are optional components andadd to proper functioning of the system but not strictly necessary as inthe specifically shown configuration, as modifications of theconfiguration are also possible.

One example of a multi-stream heat exchange units is illustrated in FIG.4 . The multi-stream heat exchange unit in FIG. 4 is of the type inwhich the coolant flows in a space 50′ of a middle section 50 that issandwiched between two heat exchange modules 65, 66. The heat istransferred from the first coolant in the middle section through walls51, 52 that delimit the space 50′ in the middle section 50 against theheat exchange modules 65, 66. This multi-stream heat exchange unit has athree-layer configuration. In the following, it will be exemplified asthe first heat exchange unit 21 that has been described above.

An example of a principle for the first heat exchange unit 21 isillustrated in FIG. 4 , wherein FIG. 4 a shows two different perspectiveviews, FIG. 4 b illustrates a semi-transparent, partially cut-away view,and FIG. 4 c a side view inside the first multi-stream heat exchangeunit 21.

The first heat exchange unit 21 comprises a first inlet 41 and a firstoutlet 42 for first coolant, and a first flow path 47 there in betweenfor flow of the first coolant from the first inlet 41 through the firstflow path 47 to the first outlet 42. The first heat exchange unit 21comprises a second inlet 43 and a second outlet 44 for cathode gas, forexample air, to provide oxygen gas to the fuel cell. A second flow path48 is provided between the second inlet 43 and second outlet 44 for thecathode gas to flow from the second inlet 43 through the second flowpath 48 to the second outlet 44. The first heat exchange unit 21comprises a third inlet 45 and a third outlet 46 for providing fuel 20to the fuel cell 15. A third flow path 49 is provided between the thirdinlet 45 and third outlet 46 for the fuel 20 to flow from the thirdinlet 45 through the third flow path 49 to the third outlet 46.

In the example of a multi-stream heat exchange unit in FIG. 4 , and bestseen in FIG. 4 c , the first flow path 47 of the first coolant is in amiddle section 50 between a first wall 51 and a second wall 52, wherethe first wall 51 is provided between the middle section 50 and thefirst heat exchange module 65, and the second wall 52 is providedbetween the middle section 50 and the second heat exchange module 66. Asthe walls are made of metal, for example aluminum, a proper heattransfer is provided from the first coolant to the cathode gas and tothe fuel in the oppositely provided heat exchange modules 65, 66.

The second flow path 48 for the cathode gas, for example air, is betweenmultiple corrugated plates in order to provide a large metal surfacethat transfers heat to the cathode gas. The corrugated plates alsocreate turbulence, which is another advantage.

The third flow path 49 for the fuel 20 is meander-shaped for a betterheat transfer as compared to a single straight path along the first wall51. The tube for the third flow path 49 is increasing in diameter alongthe flow path 49, which is advantageous because the fuel 20 increases involume during evaporation.

FIG. 5 illustrates a different principle of a multi-steam heat exchangeunit in which the coolant is diverted into two opposite parts of theheat exchange unit, for example into two opposite heat exchange modules.This implies that a first part of the coolant is flowing into onesection of the heat exchange unit, or at least predominantly, exchangingenergy with fluid in that section of the heat exchange unit, while asecond part of the coolant is flowing into another section of the heatexchange unit and only, or at least predominantly, exchanging energywith fluid in that section. In the following, it will be exemplified asthe second heat exchange unit 22 that has been described above, with athird and fourth heat exchange module. However, the principle could alsobe applied for the first heat exchange unit 21.

The second heat exchange unit 22 comprises a fourth inlet 53 and afourth outlet 54 for flow of the second coolant from the fourth inlet 53through the second heat exchange unit 22 and to the fourth outlet 54. Italso comprises a fifth inlet 55 and fifth outlet 56 for flow of thefirst coolant through the second heat exchange unit 22. It comprises asixth inlet 57 and a sixth outlet 58 for flow of exhaust gas through thesecond heat exchange unit 22. It further comprises a seventh inlet 61for inlet of reformer gas.

When comparing with FIG. 1 , both exhaust gas 27 from the fuel cell 15and burner reformer gas 37 as well as gas from the startup heater 5 flowthrough corresponding tubes to the second heat exchange unit 22. Asmentioned before, these two gases 27, 37 can be combined prior toleading the gases 27, 37 into the second heat exchange unit 22. However,in the exemplified second heat exchange unit 22 of FIG. 5 , the twogases 27, 37 are combined inside the second heat exchange unit 22. Theexhaust gas 27 from the fuel cell 15 enters sixth inlet 57, and thereformer burner gas 37 enters the seventh inlet 62. The sixth flow path61 for the exhaust gas 27 is from the sixth inlet 57 into the secondheat exchange unit 22 and towards opening 64. The seventh flow 63 pathfor the reformer burner gas 37 it through the seventh inlet 62, then toa further opening 68 after which the reformer burner gas 37 mixes withthe exhaust gas 27, which also flows to opening 64, from which the flowpath 61′ is a combined sixth and seventh flow through opening 64 andthen into tube 67 towards sixth outlet 58.

The second coolant flows into the second heat exchange unit 22 throughthe fourth inlet 53 along the fourth flow path 59 and out of the secondheat exchange unit 22 through the fourth outlet. Notice that the secondcoolant is diverted into two flow paths 59A, 59B, one for heat exchangewith the first coolant in one section of the second heat exchange unit22 and one for heat exchange with the mix of exhaust gas 27 and reformerburner gas 37 in another section of the second heat exchange unit 22.For efficient heat exchange, two sets 69A, 69B of a plurality ofcorrugated sheets 70 are provided.

REFERENCE NUMBERS

-   1 fuel cell system-   2 methanol supply tank-   3 methanol-   4 tubes-   4 a methanol supply tube to startup heater 5-   4 b, 4 c methanol supply tube to reformer burners 7-   4 d methanol supply tube to evaporator in heat exchange unit 21-   5 startup heater-   6 valve-   7 reformer burners-   8 reformers-   9 water-   10 mixing point for water and methanol-   11 syngas-   12 anode side-   13 path for first coolant through fuel cell 15-   14 cathode side-   15 fuel cell-   16 air-   17 environment-   18 first cooling circuit-   18A first branch of first cooling circuit 18-   18B second branch of first cooling circuit 18-   18C third branch of first cooling circuit 18-   19 coolant inlet of fuel cell 15-   20 fuel, ex. methanol/water mix-   21 first heat exchange unit-   22 second heat exchange unit-   23 housing-   25 second cooling circuit-   26 radiator-   27 exhaust gas from fuel cell-   28 liquid/gas separator-   29 water storage tank/reservoir-   30 exhaust-   32 voltage converters-   33 DC/AC converters-   34 a methanol filter-   34 b water filter-   35 valve in branch 18C-   36 valve in branch 18B-   37 gas from startup heater 5 and from reformer burner 7-   38 pump-   39 branch point between branches 18A and 18B downstream of second    heat exchange unit 22-   40 branch point between 18A and 18B upstream of second heat exchange    unit 22-   41 first inlet of first heat exchange unit 21 for first coolant-   42 first outlet of first heat exchange unit 21 for first coolant-   43 second inlet of first heat exchange unit 21 for cathode gas, ex.    air-   44 second outlet of first heat exchange unit 21 for cathode gas, ex.    air-   45 third inlet of first heat exchange unit 21 for fuel-   46 third outlet of first heat exchange unit 21 for fuel-   47 first flow path for first coolant-   48 second flow path for air-   49 meander-formed third flow path for fuel-   50 space forming a middle section with coolant flow-   51 wall between middle section and evaporator-   52 wall between middle section and air heater-   53 fourth inlet of second heat exchange unit 22 for second coolant-   54 fourth outlet of second heat exchange unit 22 for second coolant-   55 fifth inlet of second heat exchange unit 22 for first coolant-   56 fifth outlet of second heat exchange unit 22 for first coolant-   57 sixth inlet of second heat exchange unit 22 for exhaust gas-   58 sixth outlet of second heat exchange unit 22 for exhaust gas-   59 fourth flow path for second coolant-   60 fifth flow path for first coolant-   61 sixth flow path for exhaust gas-   61′ combined sixth 61 and seventh flow path 63 for mix of exhaust    and reformer gas-   62 seventh inlet for reformer gas-   63 seventh flow path for reformer gas-   64 opening for sixth and seventh flow path-   65 first heat exchange module in first heat exchange unit 21-   66 second heat exchange module in first heat exchange unit 21-   67 tube between opening 64 and sixth outlet 58-   68 further opening for seventh flow path 63

The invention claimed is:
 1. A fuel cell system, comprising: a fuel cellcomprising an anode side and a cathode side and a proton exchangemembrane therein between for transport of hydrogen ions from the anodeside to the cathode side through the membrane during operation; acathode gas supply, for example air supply, for supplying oxygen gas tothe cathode side, the cathode gas comprising oxygen gas, a cathode gasheating system, for example an air heating system, for increasing thetemperature of the oxygen gas prior to supplying the cathode gas to thecathode side; a first cooling circuit for circulating first coolantthrough the fuel cell for adjusting the temperature of the fuel cellwith the first coolant; a reformer comprising a catalyser enclosed byreformer walls and configured for catalytic conversion of fuel tosyngas, wherein the reformer is conduit-connected to the anode side ofthe fuel cell for provision of syngas to the fuel cell; an evaporatorconfigured for evaporating liquid fuel and conduit-connected to thereformer for provision of the evaporated fuel to the reformer; a liquidfuel supply conduit-connected to the evaporator for providing liquidfuel to the evaporator; a reformer burner for heating the catalyserinside the reformer by heat transfer through the reformer walls;characterised in that the cathode gas heating system and the evaporatorare combined in a single compact first heat exchange unit whichcomprises a first housing and which is configured for transfer ofthermal energy from the first coolant inside the first housing to boththe cathode gas and the fuel.
 2. The fuel cell system according to claim1, wherein the first heat exchange unit comprises: a first inlet that isconduit-connected to the first coolant circuit at a downstream side ofthe fuel cell for receiving the first coolant from the fuel cell afterthe first coolant has traversed the fuel cell; a first outlet for outletfor the first coolant after heat transfer from the first coolant to thecathode gas and the fuel; a second inlet that is conduit-connected tothe cathode gas supply, for example air supply, for receiving oxygen gasfor the cathode side of the fuel cell; a second outlet pipe connected tothe cathode side of the fuel cell for providing heated cathode gas tothe fuel cell; a third inlet that is conduit-connected to the fuelsupply for receiving fuel, for example a methanol/water mix; a thirdoutlet conduit-connected to the anode side of the fuel cell forsupplying evaporated fuel to the anode side of the fuel cell; whereinthe first heat exchange unit comprises a first, second and third flowpath separated from each other by thermally conducting walls, where inthe first flow path is between the first inlet and the first outlet andflow-connected with the first inlet and the first outlet, the secondflow path is between the second inlet and flow-connected with the secondinlet and the second outlet, and the third flow path is between thethird inlet and the third outlet and flow-connected with the third inletand the third outlet.
 3. The fuel cell system according to claim 1,wherein the cathode gas heating system is provided as a first heatexchange module in the second flow path between the second inlet and thesecond outlet, and the evaporator is provided as a second heat exchangemodule in the third flow path between the third inlet and the thirdoutlet, wherein there is provided a space inside the first housing, thespace being sandwiched between the first and the second heat exchangemodules and being configured for flow of the first coolant in the spacefor transferring heat from the first coolant to both the first and thesecond heat exchange modules simultaneously on either side of the space.4. The fuel cell system according to claim 3, wherein the first and thesecond heat exchange modules are provided in parallel with respect toeach other with the space for the flow of the first coolant in between.5. A method of operating a fuel cell, the fuel cell system comprising: afuel cell comprising an anode side and a cathode side and a protonexchange membrane therein between for transport of hydrogen ions fromthe anode side to the cathode side through the membrane duringoperation; a cathode gas supply, for example air supply, for supplyingoxygen gas to the cathode side, the cathode gas comprising oxygen gas, acathode gas heating system, for example an air heating system, forincreasing the temperature of the oxygen gas prior to supplying thecathode gas to the cathode side; a first cooling circuit for circulatingfirst coolant through the fuel cell for adjusting the temperature of thefuel cell with the first coolant; a reformer comprising a catalyserenclosed by reformer walls and configured for catalytic conversion offuel to syngas, wherein the reformer is conduit-connected to the anodeside of the fuel cell for provision of syngas to the fuel cell; anevaporator configured for evaporating liquid fuel and conduit-connectedto the reformer for provision of the evaporated fuel to the reformer; aliquid fuel supply conduit-connected to the evaporator for providingliquid fuel to the evaporator; a reformer burner for heating thecatalyser inside the reformer by heat transfer through the reformerwalls; characterised in that the cathode gas heating system and theevaporator are combined in a single compact first heat exchange unitwhich comprises a first housing and wherein the method comprisestransferring thermal energy from the first coolant inside the firsthousing to both the cathode gas and the fuel.
 6. The method according toclaim 5, wherein the first heat exchange unit comprises: a first inletconduit-connected to the first coolant circuit at a downstream side ofthe fuel cell for receiving the first coolant from the fuel cell afterthe first coolant has traversed the fuel cell; a first outlet for outletfor the first coolant after heat transfer from the first coolant to thecathode gas and the fuel; a second inlet conduit-connected to thecathode gas supply, for example air supply, for receiving oxygen gas forthe cathode side of the fuel cell; a second outlet pipe connected to thecathode side of the fuel cell for providing heated cathode gas to thefuel cell; a third inlet conduit-connected to the fuel supply forreceiving fuel, for example a methanol/water mix; a third outletconduit-connected to the anode side of the fuel cell for supplyingevaporated fuel to the anode side of the fuel cell; wherein the firstheat exchange unit comprises a first, second and third flow pathseparated from each other by thermally conducting walls, where in thefirst flow path is between the first inlet and the first outlet andflow-connected with the first inlet and the first outlet, the secondflow path is between the second inlet and the second outlet andflow-connected with the second inlet and the second outlet, and thethird flow path is between the third inlet and the third outlet andflow-connected with the third inlet and the third outlet; wherein themethod comprises: receiving first coolant through the first inlet from adownstream side of the fuel cell after the first coolant has traversedthe fuel cell; receiving cathode gas through the second inlet from thecathode gas supply, for example air supply, and transferring thermalenergy from the first coolant in the first flow path to the cathode gasin the second flow path, and then releasing the cathode gas through thesecond outlet for providing heated cathode gas to the fuel cell;receiving fuel through the third inlet from the fuel supply andtransferring thermal energy to the fuel in the third flow path and thenreleasing evaporated fuel through the third outlet for supplying theevaporated fuel to the anode side of the fuel cell.
 7. The methodaccording to claim 5, wherein the cathode gas heating system is providedas a first heat exchange module in the second flow path between thesecond inlet and the second outlet, and the evaporator is provided as asecond heat exchange module in the third flow path between the thirdinlet and the third outlet, wherein there is provided a space inside thefirst housing, the space being sandwiched between the first and thesecond heat exchange modules, wherein the method comprises providingflow of the first coolant in the space and transferring heat from thefirst coolant to both the first and the second heat exchange modulessimultaneously on either side of the space.
 8. The method according toclaim 7, wherein the first and the second heat exchange modules areprovided in parallel with respect to each other and with the space inbetween, wherein the method comprises providing flow of the firstcoolant in the space.
 9. The method according to claim 5, wherein thefuel cell is a high temperature polymer electrolyte membrane HT-PEM fuelcell, and wherein the method comprises operating the fuel cell at atemperature in the range of 120-200° C.